Abstract
True green pigments in the animal kingdom are scarce and are almost invariably porphyrinoids. Endogenous porphyrins resulting from the breakdown of haem are usually known as “bile pigments”. The pigmentation of intertidal Polychaeta has long gained attention due to its variety and vivid patterning that often seems incompatible with camouflage, as it occurs with Eulalia viridis, one of the few truly green Polychaeta. The present study combined UV and bright-field microscopy with HPLC to address the presence and distribution of pigments in several organs. The results showed two major types of porphyrin-like pigments, yellowish and greenish in colour, that are chiefly stored as intraplasmatic granules. Whereas the proboscis holds yellow pigments, the skin harbours both types in highly specialised cells. In their turn, oocytes and intestine have mostly green pigments. Despite some inter-individual variation, the pigments tend to be stable after prolonged storage at −20 °C, which has important implications for future studies. The results show that, in a foraging predator of the intertidal where melanins are circumscribed to lining the nervous system, porphyrinoid pigments have a key role in protection against UV light, in sensing and even as chemical defence against foulants and predators, which represents a remarkable adaptive feature.
Highlights
Most animals owe their green colour to complex chromatophores that filter visible light sequentially, rather than through biosynthesis of green pigments or their incorporation from photosynthetic organisms
The chlorin named bonellin from the Polychaeta (Echiura) Bonellia viridis was originally believed to be a derivative from chlorophyll, but later concluded that it is, an endogenous, unique chlorin[9]
Besides the role of haem porphyrins in gas exchange, inclusively in invertebrates[5], their ability to absorb UV light can be relevant to intertidal marine organisms as it may confer protection from sunlight, but this ecological role of pigments is not really explored
Summary
Most animals owe their green colour to complex chromatophores that filter visible light sequentially, rather than through biosynthesis of green pigments or their incorporation from photosynthetic organisms. Two important examples of each case are described in the Polychaeta: the exogenous chaetopterin identified in Chaetopterus variopedatus mid-gut as a chlorophyll derivative resulting from detritivore feeding[6], and the endogenous biliverdins, such as those identified in Hediste diversicolor[7] Regardless of their source, these green pigments share their tetrapyrrolic nature. Free porphyrins like some bile pigments, specially biliverdin, are reported to be strongly photodynamic and induce genotoxicity whereas anti-mutagenic and anti-oxidative stress properties are described as well[16,17,18] With these notions, the green pigments of Bonellia and Chaetopterus already began receiving particular attention due to their properties. In line with the interest in porphyrins as potential photosensitisers and their physiological role as protective pigments, the aims of this study are: i) to evaluate the diversity and distribution of the pigments along E. viridis body, ii) to infer their basic spectroscopic properties and stability and iii) to contribute to understand the pigments’ adaptative value and their main functions
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